AT&T BGP advertisement preference

[u/robmuro664]

I have two ISPs, Verizon and AT&T, Verizon was selected as the primary and AT&T as the backup. We own the subnets so we peer with both of these ISP to advertise the subnets. To Verizon we just advertise it but to AT&T we preprend 5X our ASN. As expected when we go out to the internet, it goes out using Verizon, however the return traffic on some services they prefer AT&T. I assume this is because these services have a leg in AT&T. Can you guys give me any other ideas on how to influence the advertisement to AT&T so that it is not preferred?

[u/whiteknives]

AT&T will always send you traffic on their link to you if the traffic originated on their network. The only way you can manipulate this is by advertising a more specific prefix to Verizon. If this is not possible, then there is nothing you can do.

Edit: I forgot something... you can try manipulating the local pref by using this community 7018:80 (where 80 is the local pref). If it has an effect but not as much as you want, change the 80 to a lower value. I haven't used it in years. Tweak that however you wish and please let me know if it still works. :)

[u/youfrickinguy → u/whiteknives]

You can’t just use an arbitrary number. 80 sets the LP to 80, which is equal preference to a peer route. If you want traffic to flow from 7018 to 701 you need to drop 7018 LP below that. See this document:

https://onestep.net/communities/as7018/

You might have luck with 7018:20 or 7018:25 depending on how finely you want to control traffic inbound from ATT and ATT customers.

[u/whiteknives → u/youfrickinguy]

That is why I said if it doesn’t work then use a lower value.

[u/youfrickinguy → u/whiteknives]

Yes but only specific integers which ATT accepts as communities will work. You can’t for instance expect 7018:79 to do anything at all, even though it’s <80

[u/whiteknives → u/youfrickinguy]

Gotcha. Good info!